Cellular mobile communications networks have the peculiarity of featuring a plurality of so-called “network cells”, where by the term cell there is intended the set of geographical “points”, “pixels”, in jargon (practically, small areas of, e.g., 50 m by 50 m, or 10 m by 10 m), which are covered by the radio electromagnetic signal irradiated by a generic common antenna. Cellular mobile communications networks thus provide coverage of a determined geographic region by means of the plurality of network cells.
Among the different types of cellular mobile communications networks, some networks have a radio access front end exploiting the CDMA access scheme to the shared (radio) communication medium. This is for example the case of third-generation cellular mobile communications networks, currently being deployed. One of the third-generation mobile communications standards is the so-called Universal Mobile Telecommunications System (UMTS), which is in particular the standard that has been adopted by operators in Europe.
CDMA is a technique of accessing a shared communications medium according to which a same frequency band (a “channel”) is assigned simultaneously to all the requesting users; the discrimination among different signals intended for different users is accomplished by exploiting a coding scheme, according to which different codes are assigned to different users, and the signals directed thereto are coded using the respective codes.
The codes assigned to different users and exploited for coding the signals directed thereto need to be “orthogonal”.
The coding process comprises a so-called “spreading” operation, according to which the bandwidth of the original (uncoded) signal is widened, in particular spread over a larger bandwidth, at the same time reducing the average signal power. The spreading is achieved by coding the signal using a code that contains a higher number of symbols than the number of bits to be transmitted; the coded signal thus has a symbol rate (the so-called “chip rate”) higher than the bit rate of the original signal.
A “scrambling” process is further implemented, by applying a “scrambling code” to the signal after the spreading operation, with the purpose of scrambling the different symbols. The scrambling operation does not increase the signal bandwidth (the symbol rate is not changed compared to the chip rate of the spread signal), and can be viewed as the addition of a “color” to the signal, that allows identifying the transmission source. Particularly, in downlink (i.e., from the radio base station to the mobile terminal), the scrambling process allows distinguishing the signals within a given network cell from the signal within a different cell: to this purpose, different scrambling codes are used in different cells, in particular if such cells are neighboring.
The adoption of the CDMA access scheme has an impact on the “handover” procedures, by which, generally speaking, there is intended the set of procedures that makes it possible to keep active a service provided to a generic mobile user even when the user moves. In particular, in the CDMA access scheme a mobile user may exploit a same radio channel in different cells; thus, the passage of responsibility (handover) of a given mobile user from one network cell to another adjacent thereto (typically, in consequence of the movement of the mobile user through the geographic area) can be handled by keeping the communication with the user active on the same channel. In particular, thanks to the fact that the signals irradiated by different sources (different antennas corresponding to different cells) are distinguishable because of the use of different scrambling codes, a mechanism referred to as “soft-handover” (relying on a particular type of receiver in the User Equipment—UE—, called “Rake”) allows the mobile user's terminal to decode signals coming from, and thus to exchange information with, two or more different antennas or, more generally, with different radio base stations. In particular, thanks to the soft-handover mechanism, the UE can distinguish between signals issued by different radio base stations, i.e. by different cells, by looking at the different signal color. These areas with coverage from two or more cells are referred to as “soft-handover areas” or “macrodiversity areas”. More generally, the term “macrodiversity” is used to identify the process allowing the UE connection through more than one Base Station/Access Point at the same time for the same connection in progress. The different network cells to which the UE is simultaneously connected form the so-called “Active Set” (AS).
As known, in the UMTS the set of scrambling codes used in downlink is represented by the Gold codes featuring low self-correlation and cross-correlation. The length of the Gold code for the UMTS system is in principle equal to eighteen bits, for a total of 218−1=262,143 different codes. However, in order to keep the receiver not too complex, only a fraction of such a vast set of codes is effectively exploited in practical implementations. Specifically, the standard currently prescribes that the number of usable codes in UMTS networks is limited to a pool of 8,192 different codes. The pool of 8,192 usable scrambling codes is subdivided into 512 groups, each group including 16 codes, where one of the sixteen codes takes the role of a so-called “Primary Scrambling Code” (PSC), and the remaining 15 codes of the group are “secondary scrambling codes”.
When planning an UMTS network, or a particular regional area thereof, a unique PSC (and, consequently, the associated 15 secondary scrambling codes associated to that PSC) has to be assigned to each cell of the area under planning, the PSC being chosen among the available 512 PSCs.
The pool of 8,192 scrambling codes available for use is further considered as subdivided into 64 code groups of 128 codes each, where, within the generic code group, eight codes among the 128 codes are primary scrambling codes; thus, the pool of 8,192 available scrambling codes includes 64 code groups, each one including eight primary scrambling codes (and associated secondary scrambling codes).
In downlink, the primary scrambling code plays a role in the procedure called “cell search”, which includes the set of operations that allow the UE synchronize to the network and decode the control channels of the network cell wherein it is located. Specifically, the UE invokes the cell search procedure in either one of two cases:                whenever the UE is turned on and has to register to the network for the first time (after a previous de-registration in consequence to a turn off); or        for purposes of measuring the common channels of the adjacent cells, with the aim of updating the AS of different network cells to which the mobile terminal is connected (a procedure called “cell reselection”).        
The cell search procedure has an impact on the UE performance: depending on the complexity of the operations to be performed, the UE battery charge consumption, as well as the time required by the UE for synchronizing and decoding the control channel (the so-called “Broadcast Control Channel”—BCH) over which the network information travels, vary. In particular, the cell search procedure impact on the battery charge consumption is higher when the procedure is performed in support of the cell reselection procedure, because such operation is carried out more frequently compared to the initial synchronization of the UE upon turning it on.
The assignment of scrambling codes in downlink can be effected by means of planning algorithms. In particular, the scrambling code assignment should satisfy a PSC re-use requirement, according to which unique PSCs, within the set of 512 available PSCs, have to be univocally assigned to neighboring cells belonging to the geographic area being planned: this is essential for a correct implementation of the soft-handover, because the generic UE located in a macrodiversity area should be capable of discriminating between signals irradiated by different antennas (i.e., signals by different cells in the so-called “monitored set” of the UE).
In the paper by Y. H. Jung and Y. H. Lee, “Scrambling code planning for 3GPP W-CDMA systems”, IEEE VTC2001 Spring, Rhodes, Greece, May 2001, an M×M matrix C=[cij] (where M is the number of network cells) is defined, called “compatibility matrix”, wherein the generic matrix element cij is a constant equal to either one or zero, depending on whether or not the distance between the i-th and j-th cells is or not less than a so-called “reuse distance”, which is the minimum distance between cells that can have an identical code set. Moreover, a set S of available code indexes S={1, 2, . . . , z} is introduced. The scrambling codes are then assigned in the respect of two constraints: first, only one scrambling code set is assigned to a cell; second, the code index separation between the i-th and j-th cells should be greater than or equal to cij.